He sent a letter to ECan in February asking why nothing had been done to stop wallabies crossing the boundary.
In a response to Mr Brown's letter, ECan chief operating officer Nadeine Dommisse said options for managing the spread of wallabies were being explored.
But she said some of the sightings Mr Brown mentioned were old or from anecdotal sources.
"Inspections by both Otago and Canterbury regional council staff from the air and on the ground have not found any evidence of wallabies spreading across the Hawkdun Range."
Mr Brown called the response "an absolute joke".
"[ECan] are just running scared."
Asked to comment further, ECan senior manager service delivery Nick Daniels said the council stood by what it said in the letter.
"Over the past 15 years there have been isolated incidences of wallabies appearing all over the South Island.
"To date, all evidence points to these being the result of intentional relocations of individual animals rather than the spread and establishment of breeding populations.
"If Mr Brown has additional evidence of wallabies south of the Waitaki River, we would welcome the opportunity to review this with Mr Brown and the Otago Regional Council."
Mr Daniels said ECan had recently completed its new Regional Pest Management Plan and it included updated wallaby-control programmes.
Mr Brown said his group had "no hassle" working with the Otago Regional Council.
Otago Regional Council environmental monitoring and operations director Scott MacLean confirmed the council contracted Maniototo Pest Control to respond to wallaby reports in the area.
"Maniototo is a gateway to the rest of Otago, which is why it's particularly important to get on top of them in this area."
Mr McLean said it was difficult to say whether a breeding population existed in Otago, as the numbers seen and reported were still low.
"There is, however, an increasing number of sightings being reported through to us. [Regional council] staff are working closely with Environment Canterbury on cross-boundary issues."
He said about a third of the sightings in Otago were in the Maniototo.
The three most common areas for reported wallaby sightings in Otago were Maniototo, Tarras-Lindis area and North Otago.
tom.kitchin@odt.co.nz